Ethane ships, a new breed of gas carrier, are spurring technical innovation in the drive for safe, reliable and efficient delivery of this US shale byproduct
Fortunately for chemical manufacturers worldwide, the US shale-gas revolution is producing more ethane, a vital petrochemical feedstock, than can be utilised domestically.
Several global chemical players have signed up to purchase volumes of surplus US ethane so they too can enjoy the benefits of competitively priced feedstock.
To date seaborne ethane has hardly registered in gas-carrier logbooks. Traffic has been limited to some North Sea movements, between two loading and two discharge ports in a handful of small, semi-pressurised/fully refrigerated (semi-ref), ethylene-capable gas carriers.
US ethane exports have sent naval architects back to the drawing board in the drive for new designs of gas ships able to transport the required volumes of ethane in a safe, reliable and economical manner.
To date 26 ethane carriers have been contracted to lift US ethane. The orders comprise 15 ships in the 27,500-36,000m3 size range for transatlantic charter work and 11 very large ethane carriers (VLECs) of 86-87,000m3 for the carriage of cargo to China and India. The first five transatlantic vessels are already in service.
The four 35,000m3 ethane carriers being built by the Jiangnan yard in China for Navigator Gas and the three 36,000m3 vessels that Hartmann has under construction at Sinopacific in a partnership with Norwegian shipowner Ocean Yield are the largest semi-ref gas carriers ever ordered.
The Type C pressure vessel tanks on these ships are heavy units and do not optimise the use of the underdeck cargo-carrying space to the same extent as gas ships with prismatic tanks. Nevertheless, Type C tanks are robust and accommodate a wider range of cargo temperatures and pressures.
The cargo tanks on the Navigator Gas ships are of the popular bilobe configuration, whereas those on the Hartmann vessels are of its new Star Trilobe design. The German owner points out that vessels with Star Trilobe tanks provide 30 per cent more cargo-carrying capacity than bilobe tank ships of the same hull dimensions.
But the Navigator Gas and Hartmann/Ocean Yield ethane carriers will not hold the record as the largest semi-ref gas carriers for long.
Five of the 11 VLECs are 86,000m3 vessels also under construction as semi-ref ships with Type C tanks. The quintet is on order at Sinopacific for United Ethane Carriers (UEC), a Jaccar/Hartmann joint venture.
Ethane has a vapour pressure of 3.85 MPa at 21.1˚C, a boiling point of -88.5˚C and a specific gravity of 0.54. For transport by sea ethane must be either fully refrigerated or have both temperature and pressure controlled. Carriage in a fully pressurised gas ship is not a viable option as the shell thickness required would create a prohibitively heavy cargo tank.
The fully refrigerated option comes into its own when larger cargo volumes are involved. For this reason the specification of Type C tanks for the 86,000m3 UEC ships caught the industry by surprise. Like the Hartmann/Ocean Yield ethane carriers, the UEC ships will have Star Trilobe tanks.
The remaining VLECs, a set of six 87,000m3 vessels under construction at Samsung for Reliance of India, will be built to the fully refrigerated design. The Reliance ships will be fitted with the Gaz Transport & Technigaz (GTT) Mark III membrane tank containment system fitted on more than 150 LNG carriers in service, albeit a more robust version to accommodate the denser cargo.
To be operated and managed by Mitsui OSK Lines (MOL) of Japan, the Reliance ships will be the largest gas tankers that are not LNG carriers.
The first of the new breed of ethane carriers to be contracted, in January 2013, were four 27,500m3 vessels for Evergas. The shipowner later upped the order for these Dragon-class vessels to eight ships, all for charter to Ineos.
The Evergas ships are designed as multigas carriers, able to carry LNG, LPG and chemical gases as well as ethane, and are fitted with Wärtsilä four-stroke dual-fuel engines that run on LNG as well as marine gas oil and fuel oil.
Early in the construction phase Ineos weighed up the pros and cons of bunkering the Dragon-class ethane ships with LNG and found the prospect, at this early stage in the evolution of small-scale LNG, logistically and commercially challenging. The LNG bunker offers that came in were based on term take-or-pay commitments and had attendant high infrastructure fees.
Ineos felt that a better option would be to run the propulsion system on ethane cargo boil-off gas (BOG) and to use the two deck tanks on each ship to carry additional quantities of ethane. Wärtsilä and Evergas were requested to make the necessary modifications and to test the viability of the new configuration.
In spring 2015, at the request of Evergas, Bureau Veritas assessed the use of Wärtsilä medium-speed, dual-fuel engines using ethane as fuel. In doing so, the class society for the Dragon-class ships verified the viability of the option.
According to Carlos Guerrero, BV’s business development manager for gas carriers and oil tankers, “The recently revised International Gas Carrier (IGC) Code permits the use of ethane as propulsion system fuel provided it guarantees the same level of safety as the burning of natural gas. In addition the approval is subject to the ship’s flag administration.”
Evergas has also introduced another gas carrier innovation with the Dragon ships – the addition of a tank head condenser. These units condense ethane vapours from the cargo tanks against an LNG or ethane liquid flow that is in the process of being gasified for use as fuel. The condensers also help reduce the call on the ship’s reliquefaction system to keep the cargo cooled down, which in turn reduces overall power requirements and fuel consumption.
The ethane carrier orders following the Dragon ships reflect the fact that low-speed propulsion systems have been gaining favour in the gas ship sector in recent years. For example, the three 36,000m3 Hartmann/Ocean Yield ships will be fitted with the world’s first ethane-burning, two-stroke diesel engine.
Each vessel will be powered by a Mitsui-MAN B&W 7G50ME-C9.5-GIE engine. The operation of these dual-fuel MAN engines on ethane entails high gas injection pressures, of around 630 bar (63,000 kPa). Ethane-burning ME-GIE engines have also been specified for the VLEC quintet building for UEC.
The 36,000m3 ethane carriers are being built to what is termed the EcoStar36K design which features a Svelte bow arrangement and the accommodation/navigation superstructure forward. Hartmann states that the configuration, a gas carrier first, yields reductions in fuel consumption and atmospheric emissions by improving the vessel’s seakeeping performance at higher transit speeds.